The present teachings relate to a method and system for creating a thermoelectric cooling device capable of transporting large heat loads across significant temperature differentials while maintaining a large coefficient of performance.
Thermoelectric coolers operate by the Peltier effect (which also goes by the more general name thermoelectric effect). The device has two sides, and when DC current flows through the device, it brings heat from one side to the other, so that one side gets cooler while the other gets hotter. The “hot” side is attached to a heat sink. Two unique semi-conductors, one n-type and one p-type, are placed thermally in parallel to each other and electrically in series and then joined with a thermally conducting/electrically insulating plate on each side to form a couple. When a voltage is applied to the free ends of the two semiconductors, there is a flow of DC current across the junction of the semi-conductors causing a temperature difference.
Thermoelectric coolers have long been noted for their compact construction, high reliability, and clean, quiet operation. Due to recent reductions in price and improvements in performance, thermoelectric refrigeration units are now found in consumer markets (coolers, refrigerators, etc.). Thermoelectric coolers (TECs), are, however, inefficient devices requiring large amounts of current to provide a refrigerant effect. Even modest improvements in performance would increase the market potential of thermoelectric cooling, expanding its role in commercial, military, and aerospace applications.
The limitations on refrigeration are due both to the available materials and to the inherent geometry of TEC devices. The cooling, of course, is provided via the Peltier effect, where the heat absorption by electrons as they jump to higher energy levels provides a cooling effect. At the opposite end of the Peltier material, the electrons return to a lower energy level, releasing energy and increasing the temperature at that location. Thus a temperature differential across the material is created. Unfortunately, since the Seebeck coefficients of existing materials are small, a large amount of current is required to create a significant refrigeration effect.
TEC elements within coolers are wired in series so that each junction sees the same current, which argues for designing the elements with short sections of material to reduce voltages and ohmic losses. Conduction from the hot end to the cold end of each leg is increased, however, as the leg is shortened, counteracting the energy absorption at the cold junction.
The ideal material for TEC coolers would have a high Seebeck coefficient, a low electrical resistance, and a low thermal conductivity. All currently-available materials suffer compromises among these three criteria. Since the ideal material does not yet exist (although there are ongoing efforts to significantly improve TEC materials), one must consider design solutions that circumvent material limitations and provide increased refrigeration performance.
In a conventional TEC cooler, depending on the current, the hottest point lies at the hot side or between the hot side and the midpoint of the legs. Typically, half of the ohmic heat generated will be absorbed at each junction of the leg, so increasing the thermal conduction from the midpoint to the hot side of the leg will also increase thermal conduction to the cold side of the leg, and no net benefit will result. In a conventional TEC cooler, ohmic loss causes temperature backflow to the cold side that limits the thermal transfer efficiency.
There is a need for TEC cooler designs with better thermal transfer efficiency.